This is a graph of voter participation, not voter turnout, for voting districts in Allegheny County. Voter turnout is ballots cast divided by registered voters. Voter participation here is ballots cast divided by the voting eligible (age 18 and over) population. The hypothesis that African Americans are less likely to vote would mean that the line there is sloping downward.
We can talk about exceptional elections, but the pattern in this graph is pretty typical of all local races going back many years. And yes, it is all more than one dimensional, but still pretty baseless to just say minorities vote less. Some of the things most commonly believed are often the most baseless.